Further fizzing

A reader writes:

After reading your post about vinegar and bicarbonate of soda as cleaners, I've been using bicarb more to clean and deodorize things (a bicarb and water paste is pretty good for spills on fabrics and carpets - just massage it in then leave it overnight and vacuum it up the next day. Clogs the vacuum filter something fierce, though).

In the course of my experiments, I've found that if you add bicarb to near-boiling water, it fizzes. This is with plain water fresh out of the electric kettle, not water plus vinegar or anything else acidic. Add bicarb to the water, it fizzes and dissolves. Add more bicarb, more fizz. Add more hot WATER to the existing bicarb-and-water solution, and it fizzes again!

What's going on, here? I know dissolving stuff in water can change the boiling point, but I think it usually INCREASES it, and the difference isn't usually very large. Is the bicarb providing nucleation sites for boiling? Why's it still happen when the bicarb's dissolved, though? And how can it boil water that's not hot enough to boil naturally any more?


The Wikipedia article actually explains this; above 70 °C, sodium bicarbonate and various other bicarbonates decompose. In sodium bicarbonate's case, it goes from NaHCO3 to sodium carbonate (Na2CO3), water and carbon dioxide. The hotter it is, the faster this happens, and it happens in solution too.

So the fizz is still carbon dioxide bubbles, just as if you'd added bicarb to vinegar, but the source of the CO2 bubbles is different.

Psycho Science is a regular feature here. Ask me your science questions, and I'll answer them. Probably.

And then commenters will, I hope, correct at least the most obvious flaws in my answer.

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